Correct Method

Photo Source: Summerhill

Sealing Electrical Boxes:

If gasket boxes are used, then the drywall will compress the polyethylene onto the gasket providing a seal. If an extremely air tight envelope is desired, then this gasket should be further enhanced by using acoustical sealant.

If a poly pan box or piece of polyethylene sheet is used, they must be pulled to the inside of the polyethylene. Acoustical sealant is then used to seal between the layers and an approved tape is used to fasten the poly pan box to the polyethylene sheet. This will keep the seal in place long term.

The photo above shows a typical polypan box before any sealing works has been done. The insulation in this photo is very poorly installed.